1. Field of the Invention
This invention relates to methods of image capture, more particularly for methods of detecting color misregistration in image capture devices.
2. Background of the Invention
Color image capture devices typically operate by capturing primary color component signals such as red, green and blue (RGB) from a set of charge coupled devices (CCDs). The CCDs are normally arranged in the sub-scan direction. The main scan direction, the direction in which the scanning bar moves, will be referred to as the X direction and the sub-scan direction, perpendicular to the main scan direction, will be referred to as Y.
These CCDs capture the image in one pass or in three passes, one for each primary color component. Regardless of the number of passes, however, there is typically some misalignment in the RGB signals. This misalignment between colors is referred to as color misregistration. It is caused by faulty superposition of the three colors. It normally manifests itself as color fringes on the edges of the objects that were scanned, either text, graphics or drawings.
Color fringes normally appear as either cyan or magenta fringes on the edges of the scanned objects. Cyan fringes result from misregistration of the red signal, and magenta fringes result from misregistration of the green signal. The human eye does not normally detect misregistration of the blue signal, because of its low bandwidth and low contrast sensitivity.
Most often, color misregistration occurs in the Y direction. Vibration, scanning motion and the mechanical or optical design of the scanner can lead to faulty superposition of the three-color components. Several different approaches have been taken to solve this problem.
For example, some efforts have been directed at correcting the mechanical problems in the scanner by tracking registration marks. One example of these types of techniques is found in U.S. Pat. No. 5,737,003, issued on Apr. 7, 1998. In this patent, a laser scanner used to form latent images on the photoconductive belt is used to detect the position of the edge of the belt. The belt is then controlled to reduce the deviation of the belt from its path. It also includes a method for controlling the laser, and therefore the formation of the image, based upon the position of the belt.
Another of these mechanical registration techniques is found in U.S. Pat. No. 5,774,156, issued Jun. 30, 1998. The system uses several stations, each color of toner. The latent image formed by the individual scanners at the stations includes a registration area. The registration area is then aligned prior to the application of the toner. The registration area is then recharged to avoid having the registration marks attract any toner. This is repeated at each station to ensure proper positioning of the image before the latent image for the next color is formed. U.S. Pat. No. 5,760,815, issued Jun. 2, 1998, shows another method. In this patent, a fiber optic detection means is used to detect registration signals produced by a retroreflector. The light from the retroreflector is analyzed and used to adjust the registration of the belt.
Other methods have focused on optical means to correct the misregistration. An example of these types of techniques can be found in U.S. Pat. No. 4,583,116, issued Apr. 15, 1986. In this patent, the color signals are manipulated to convert them into color separation signals for cyan, magenta, yellow and black. The edges of each of the colors is then detected and manipulated to switch lighter areas with darker areas, or vice versa, to avoid streaks and other imperfections.
Several other types of techniques are used to detect color misregistration at the data level. Examples of these are found in U.S. Pat. Nos. 5,500,746, 5,907,414, 5,477,335, and 5,764,388. In U.S. Pat. No. 5,500,746, issued Mar. 19, 1996, the signals are manipulated to ensure that the dots formed are in line both in the X and Y directions for each color. The dots are resampled and repositioned as determined by line correction devices.
In U.S. Pat. No. 5,907,414, issued May 25, 1999, one of the more powerful prior art methods is shown. An image sensor used to scanning a manuscript generates signals and these signals are examined. If the examination of the signals determines that the pixel exists at an edge of a letter image, it is identified as such. These identified pixels are then adjusted in their brightness to ensure a smooth edge that was disrupted by vibration of the image sensor.
A less sophisticated but still useful technique is shown in U.S. Pat. No. 5,764,388, issued Jun. 9, 1998. In this patent, the cyan-magenta-yellow components of a pixel are analyzed. If the chrominance of the signal is less than a threshold, it is set to zero to offset an assumed color misregistration error.
However, most of these techniques are too expensive or are too inaccurate to meet current expectations for quality. The digital color imaging market has become very competitive. Peripheral image capture and printing devices such as copiers, scanners, digital cameras, fax machines and printers continue to fall in price. Expectations of their quality continue to rise. Therefore, a method is needed that solves color misregistration at low-cost but with high accuracy.
One aspect of the invention is a method for detecting color misregistration in image data. Input image data is buffered as color space data. The color space data is then transferred to vector space. An examination window around a current pixel is established. Background and foreground pixels in this window are determined. The current pixel is examined to determine if it is on an edge of a scanned object, either text, image or graphic. If the current pixel is on an edge, Stokes"" theorem is applied to determine the magnitude of color misregistration. If the magnitude of color misregistration is above a certain threshold, the pixel is identified as having color misregistration.
Other aspects of the invention include the above method with an optional edge detection step that occurs before any detailed analysis is performed. This initial sorting can speed the process by narrowing the number of pixels that need to undergo the detailed analysis.